The invention relates to a getter arrangement for providing a getter spot on a getter surface within an evacuated space by evaporating getter material arranged near the getter surface, the getter arrangement including getter means situated outside the evacuated space for within this space in the region of the getter material generating a heating power for evaporating the getter material, the getter arrangement further including a control unit for controlling the heating power, the control unit having detection means for detecting the presence of the getter spot on the getter surface.
Such a getter arrangement is disclosed in the "Abstract" of the Japanese Patent Application number 58-247309, publication number 60-143546.
The vacuum in an evacuated space can be improved by providing a getter spot therein. This is effected by positioning in the evacuated space a holder which contains a predetermined quantity of getter material to be evaporated. This holder is arranged quite near to a getter surface, that is to say the area on which the getter spot is to be provided. Generally, an inner wall of the evacuated space is chosen for this purpose. The holder, which, is for example, in the shape of a ring, can, for example, be heated inductively by placing a highfrequency induction coil near the holder, but outside the evacuated space. This induction coil is connected to a high-frequency generator.
The holder may alternatively be heated by different heating means, for example by radiating visible or non-visible laser light into the holder, the laser light being produced by a power laser.
As soon as the getter material starts to evaporate it is deposited on the wall of the evacuated space and forms a getter spot there, thereby binding the residual gases still present. The metal barium is often used as the getter material.
The above-described gettering procedure is, for example, used during the production of vacuum electron tubes. Such a tube is first evacuated and sealed thereafter. When induction heating is applied, the holder cum getter material is provided in the tube near the glass wall, to ensure that the largest possible portion of the electromagnetic flux generated by the high-frequency induction coil will be encompassed by the annular holder, so that the high-frequency heating process occurring there will be as optimal as possible.
Because of the unavoidable inaccuracies in the positioning of the annular holder containing the getter material, relative to the high-frequency induction coil, the flux encompassed by the annular holder will vary from case to case. At a substantially constant high-frequency heating powder produced by the high-frequency generator too little getter material would be evaporated in an annular holder encompassing a low amount of flux in a predetermined period of time, and in an annular holder encompassing much flux the annular holder would become too hot causing, by melting, metal particles to be deposited freely in the evacuated space so that the components present there might get contaminated. In the first case the desired quality of the getter process is not achieved, in the second case the tube might be damaged.
The holder containing the getter material can also be heated by means of laser light. Positional inaccuracies of the holder might cause a non-uniform temperature distribution at a non-recurrently chosen fixed arrangement of the laser. In that case too small a portion of the getter material would evaporate in the coolest spot of the holder, whereas the above-described disadvantages may occur in the hottest spot. In addition, inaccuracies in the shape of the wall of the evacuated space or contaminations in this wall, for example air bubbles, may effect dispersion or absorption of the laser beam as result of which less heating power is applied to the holder than was originally the intention. This also causes the problems described in the foregoing for inductive heating.
These problems might be solved by using a much more accurate positioning of the holder, by smaller tolerances in the shape of the electron tube and by chosing material of a higher grade for the glass wall of the tube. This is however a costly solution.